1. Field of the Invention
The present invention relates to a color conversion apparatus in an image output device, such as an image display device, color printer, or color scanner, and in particular, to a color conversion apparatus, and a color conversion method converting color data representing images, in accordance with the characteristics of the output device reproducing or expressing the image.
2. Prior Art
When input color data is displayed by an image display device, color conversion is performed to achieve desired color rendition or reproducibility in accordance with the conditions in which the device is used. Also in printers, color conversion is performed to achieve good color reproducibility for compensating for mixed-color property due to the fact that inks are not of pure colors, and picture quality degradation due to non-linearity of the input-output characteristics.
Two typical color conversion methods are of a table conversion method and a matrix calculation method. The table conversion method uses a memory such as a ROM for storing conversion coefficients corresponding to the input color data representing each of the colors of R, G, B, and performs color conversion in accordance with the stored conversion coefficients. An advantage of this method is that color conversion can be performed based on an arbitrary conversion characteristics.
However, it is necessary to store conversion coefficients for respective combinations of the color data, and a memory of a large capacity is required, so that implementation in an integrated circuit is difficult. Another problem is that it cannot be adapted to changes in the condition in which the device is used.
The matrix calculation method performs a matrix calculation in accordance with the following formula (24) using the color data Ri, Gi, Bi representing colors of R, G, B, to output new color data Ro, Go, Bo.
                              [                                                    Ro                                                                    Go                                                                    Bo                                              ]                =                              (            Aij            )                    ⁡                      [                                                            Ri                                                                              Gi                                                                              Bi                                                      ]                                              (        24        )            
In the above formula, Aij (i=1 to 3, j=1 to 3) are matrix coefficients determining the conversion coefficients for the color conversion.
There is a concept of “preferred color reproduction” as opposed to the concept of “exact color reproduction.” “Exact color reproduction” means exact or faithful reproduction of colors of the original, and color conversion which matches the method of generating the input color data is performed. For example, the color reproduction using standard color space, such as that of NTSC, or sRGB is performed. “Preferred color reproduction” does not necessarily coincide with “exact color reproduction,” and is a color reproduction taking into consideration the characteristics of human visual sense, and memorized colors, and produces colors preferred by human beings.
Often, “preferred color reproduction” is performed in connection with moving pictures of television images. For instance, there is a tendency that the color of the sky, or green of the lawn is often memorized as colors which are more vivid, and of a higher saturation. Accordingly, in “preferred color reproduction,” a general practice is to perform the color conversion in which the saturation or lightness (value) of the colors are increased. In “exact color reproduction” as well, it is not rare that color conversion which increases the saturation or lightness is performed. This is because, the range or gamut of the colors which can be expressed by the image display device is narrower than the gamut of the standard color space or the color space used in the generation of the image data.
FIG. 36 shows a configuration of a conventional color conversion apparatus. A color converter 1 performs color conversion on first color data R1 G1, B1 to generate second color data R2, G2, B2. A data limiter 101 limits the second color data R2, G2, B2, to output third color data which can be displayed by the image display device, not shown, provided in a succeeding stage.
In the conventional color conversion apparatus shown in FIG. 36, due to the function of the data limiter 101, the color contraction (a phenomenon in which a fine difference between colors of high lightness or saturation is eliminated (two different input color data are translated into the same color) may occur. A specific example of color contraction due to the limiting operation will next be described.
It is presumed that the first color data R1, G1, B1 is 8-bit digital data representing an integer without a sign, assuming a value of 0 to 255, and the second color data R2, G2, B2 is 10-bit digital data with a sign, assuming a value of −512 to 511. It is also presumed that the number of gradation levels which can be displayed by the image display device used for displaying the image responsive to the second color data R2, G2, B2 is 256 (expressed by 8 bits), and the data limiter 101 accordingly converts the second color data into the third color data of 8 bits.
The color converter 1 outputs the second color data by matrix calculation represented by the following formula (25).
                              [                                                    R2                                                                    G2                                                                    B2                                              ]                =                              [                                                            1.2                                                  0                                                  0                                                                              0                                                  1.2                                                  0                                                                              0                                                  0                                                  1.2                                                      ]                    ⁡                      [                                                            R1                                                                              G1                                                                              B1                                                      ]                                              (        25        )            
The matrix calculation formula (25) is for increasing the lightness of the first color data R1, G1, B1.
For two different sets of first color data with: R1=230, G1=20, B1=20; and R1=240, G1=20, B1=20, the second color data obtained as a result of the color conversion according to the formula (25) will have values: R2=276, G2=24, B2=24; and R2=288, G2=24, B3=24. The third color data output from the data limiter 101, which limits the maximum value to “255,” and input the limited data to the image display device, not shown, will have values: R3=255, G3=24, B3=24; and R3=255, G3=24, B3=24. Thus the two sets of different values representing different colors are translated into the color data of identical values, due to the limiting operation. In this way, the input or original color data representing colors of different hue, lightness or saturation (in the input or original data) are translated into and displayed as an identical color, and the difference between colors (as existed in the original image) is eliminated or diminished—this phenomenon is called color contraction.
The color contraction also occurs when a processing for enhancing the saturation is performed in the color converter 1. The following matrix calculation formula (26) is an example used to enhance the saturation of the first color data.
                              [                                                    R2                                                                    G2                                                                    B2                                              ]                =                              [                                                            1                                                                      -                    0.1                                                                                        -                    0.1                                                                                                                    -                    0.1                                                                    1                                                                      -                    0.1                                                                                                                    -                    0.1                                                                                        -                    0.1                                                                    1                                                      ]                    ⁡                      [                                                            R1                                                                              G1                                                                              B1                                                      ]                                              (        26        )            
When color conversion using the matrix calculation formula (26) is applied to two sets of different values of color data: R1=230, G1=10, B1=10; and R1=232, G1=20, B1=20, the second color data will have values: R2=228, G2=−14, B2=−14; and R2=228, G2=−5, B3=−5. The data limiter 101 limits the minimum value of the second color data to “0” in accordance with the display characteristics of the image display device. The third color data supplied from the limiter 101 to the image display device (not shown) in a succeeding stage will have values: R3=228, G3=0, B3=0; and R3=228, G3=0, B3=0. Thus, the two sets of color data of different values are output as the color data of identical values.
As has been described, a conventional color conversion apparatus, which performs processing for enhancing lightness and/or saturation of the color data, is associated with a problem of color contraction in which fine differences between colors of high lightness or saturation diminishes.